1. Field of the Invention
The present invention relates to surgical devices and methods to guide instruments that prepare the surface of bones and other tissues for implants that replace a damaged, diseased, or otherwise painful spinal facet joint.
2. Description of Related Art
Traumatic, inflammatory, metabolic, and degenerative disorders of the spine can produce debilitating pain that can have severe socioeconomic and psychological effects. One of the most common surgical interventions today is arthrodesis, or spine fusion, of one or more motion segments, with approximately 300,000 procedures performed annually in the United States. Clinical success varies considerably, depending upon technique and indications, and consideration must be given to the concomitant risks and complications. For example, Tsantrizos and Nibu have shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, Khoo and Nagata have shown that spine fusion creates increased stresses and, therefore, accelerated degeneration of adjacent non-fused motion segments. Additionally, pseudoarthrosis, as a result of an incomplete or ineffective fusion, may reduce or even eliminate the desired pain relief for the patient. Finally, the fusion device, whether artificial or biological, may migrate out of the fusion site.
Recently, several attempts have been made to recreate the natural biomechanics of the spine by use of an artificial disc. Artificial discs provide for articulation between vertebral bodies to recreate the full range of motion allowed by the elastic properties of the natural intervertebral disc that directly connects two opposed vertebral bodies.
However, the artificial discs proposed to date do not fully address the mechanics of motion of the spinal column. In addition to the intervertebral disc, posterior elements called the facet joints help to support axial, torsional and shear loads that act on the spinal column. Furthermore, the facet joints are diarthroidal joints that provide both sliding articulation and load transmission features. The effects of their absence as a result of facetectomy was observed by Goh to produce significant decreases in the stiffness of the spinal column in all planes of motion: flexion and extension, lateral bending, and rotation. Furthermore, contraindications for artificial discs include arthritic facet joints, absent facet joints, severe facet joint tropism or otherwise deformed facet joints, as noted by Lemaire.
U.S. Pat. No. Re. 36,758 to Fitz discloses an artificial facet joint where the inferior facet, the mating superior facet, or both, are resurfaced. U.S. Pat. No. 6,132,464 to Martin discloses a spinal facet joint prosthesis that is supported on the posterior arch of the vertebra. Extending from this support structure are inferior and/or superior blades that replace the cartilage at the facet joint. The Martin prosthesis generally preserves existing bony structures and therefore does not address pathologies that affect the bone of the facets in addition to affecting the associated cartilage. Furthermore, the Martin invention requires a mating condition between the prosthesis and the posterior arch (also known as the lamina) that is a thin base of curved bone that carries all four facets and the spinous process. Since the posterior arch is a very complex and highly variable anatomic surface, it would be very difficult to design a prosthesis that provides reproducible positioning to correctly locate the cartilage-replacing blades for the facet joints.
Another approach to surgical intervention for spinal facets is provided in WO9848717A1 to Villaret. While Villaret teaches the replacement of spine facets, the replacement is interlocked in a manner to immobilize the joint.
It would therefore be an improvement in the art to provide a vertebral facet replacement device and method that provides a relatively high degree of mobility in the joint, while effectively removing the source of arthritic, traumatic, or other disease mediated pain with a minimum of patient discomfort.